The present invention relates generally to improvements in television receiver tuner circuits and, more particularly to a television receiver UHF tuner circuit incorporating a MOSFET (metal oxide semiconductor field effect transistor) mixer.
A superheterodyne UHF tuning circuit for a television receiver typically comprises a tunable RF amplifier connected for coupling received UHF television signals through one or more filter circuits to the input of a mixer device. An appropriate local oscillator signal is also coupled to the input of the mixer where it is heterodyned or beat with the received radio frequency UHF television signal to develop an intermediate frequency signal for subsequent processing by the television receiver. UHF mixing devices normally comprise hot carrier diodes which impose a limitation on a large signal handling capability of the UHF tuning circuit due to cross-modulation and intermodulation interference.
Improvements in UHF channel reception for television receivers can be achieved by replacing the hot carrier diode mixer in the UHF tuner circuit with an improved mixing device such as a MOSFET. The use of MOSFET mixer in a UHF television tuner circuit results in an improvement in large signal handling capability while minimizing RF interference caused by cross-modulation and intermodulation. In addition, the use of a MOSFET mixer enables the realization of a conversion gain rather than a conversion loss as in the case of a hot carrier diode mixer. Furthermore, by virtue of the unilateral active device characteristics of a MOSFET, especially a dual-gate MOSFET, and the reduced input/output impedance variations of the MOSFET in response to variations in local oscillator pumping power, more consistent bandpass characteristics of both the RF and IF filtering circuits can be achieved to enhance the overall selectivity of the receiver and to reduce its susceptibility to RF interference.
Heretofore, various limitations have militated against the use of field effect transistors, e.g. MOSFET's , in UHF television receiver tuning circuits, especially those tuning circuits incorporating varactor diode tuning elements. In particular, the rather large parasitic input capacitance of the MOSFET together with the additional circuit capacitance introduced by the conventional capacitive coupling of the local oscillator signal to the mixer results in an insufficient tuning range across the UHF band. Also, relatively poor RF bandpass characteristics are achieved due to the mismatch between the tuning resonance of the MOSFET and the other resonant circuits of the tuner. Furthermore, due to the mismatch between the relatively high input impedance of the field effect transistor mixer, and the relatively low impedance of the varactor tuned circuit typically coupled to the input thereof, especially at the low end of the UHF band, satisfactory conversion gain over the entire UHF band cannot be achieved using conventional techniques for coupling the RF and local oscillator signals to the input of a field effect transistor mixer.
U.S. Pat. No. 4,048,598 teaches the use of an impedance transformation network, comprising a series connected capacitor and a shunt connected inductor, to achieve a proper impedance match between a varactor tuned circuit and a field effect transistor amplifier to increase the signal power transferred therebetween at the low end of the UHF band. Co-pending application Ser. No. 200,806, filed Oct. 27, 1980 now U.S. Pat. No. 4,363,135 and assigned to the assignee of the present invention, teaches the use of a bandswitchable impedance transformation network connected between a varactor tuned circuit and a field effect transistor used as a VHF mixer. However, neither of these teachings addresses the problem of using a field effect transistor as a UHF mixer nor the associated problem of devising a satisfactory method for coupling a local oscillator signal to the input of a field effect transistor UHF mixer.
It is accordingly a basic object of the present invention to provide an improved UHF tuner for use in a television receiver.
It is another object of the invention to provide an improved UHF tuner for use in a television receiver, which UHF tuner includes a field effect transistor mixer.
It is a further object of the invention to provide an improved UHF tuner for use in a television receiver, which UHF tuner includes a field effect transistor mixer and a novel circuit arrangement for coupling a local oscillator signal to the input of the field effect transistor mixer.
In accordance with one aspect of the present invention, a television receiver UHF tuning circuit includes a varactor tuned circuit coupled to an input electrode of a field effect transistor mixer through a serially connected RF coupling capacitor, a local oscillator signal being coupled to the input electrode of the field effect transistor mixer through an inductive loop whose inductance value is chosen such that proper impedance matching over the UHF band, and especially at the low end of the band, is achieved.
In accordance with another aspect of the invention, by selecting the RF coupling capacitor and the inductive local oscillator coupling loop to provide, in conjunction with the input parasitic capacitance of the field effect transistor mixer, a resonant frequency below the lowest frequency in the UHF band, local oscillator power coupling is achieved without capacitively loading the input electrode of the field effect transistor mixer enabling the varactor tuned circuit to achieve a satisfactory tuning range and proper tracking in relation to the other circuits of the tuner.